Electrolytic capacitor

ABSTRACT

An electrolytic capacitor includes a capacitor element and an electrolyte solution. The capacitor element includes: an anode foil on which a dielectric layer is formed; a cathode foil which is opposite to the anode foil; and a conductive polymer layer that is interposed between the anode foil and the cathode foil, conductive polymer layer including a conductive polymer. A conductive layer provided with a carbon layer including conductive carbon is formed on the cathode foil. The conductive polymer layer is a layer formed with use of a dispersion or a solution containing the conductive polymer. And a proportion of water in the electrolyte solution ranges from 0.1% by mass to 6.0% by mass, inclusive.

CROSS REFERENCE

This application is a Continuation Application of U.S. application Ser.No. 16/909,150 filed on Jun. 23, 2020 which is a ContinuationApplication of Ser. No. 16/569,889 (now U.S. Pat. No. 10,790,095) filedon Sep. 13, 2019, which is a Continuation Application of U.S.application Ser. No. 15/725,353 (now U.S. Pat. No. 10,453,618) filed onOct. 5, 2017, which is a continuation of International Application No.PCT/JP2016/001334, dated Mar. 10, 2016, which claims the benefit ofJapanese Application No. 2015-091448 filed on Apr. 28, 2015, the entirecontents of each are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to an electrolytic capacitor thatincludes a capacitor element having a conductive polymer layer, and anelectrolyte solution.

BACKGROUND

As small-sized, large capacitance, and low ESR (Equivalent SeriesResistance) capacitors, promising candidates are electrolytic capacitorsincluding an anode body on which a dielectric layer is formed and aconductive polymer layer formed so as to cover at least a part of thedielectric layer. In Unexamined Japanese Patent Publication No.2008-010657, proposes an electrolytic capacitor obtained by impregnatingwith an electrolyte solution a capacitor element is proposed. Theelectrolytic capacitor includes an anode foil on which a dielectriclayer is formed, a cathode foil, a separator interposed between theanode foil and the cathode foil, and a conductive polymer layer formedon surfaces of the dielectric layer, the separator, and the cathodefoil.

In Unexamined Japanese Patent Publication No. 2006-100478, it isproposed that, from the viewpoint of increasing adhesion betweenaluminum and a carbon-containing layer in a cathode of a solidelectrolytic capacitor, an interposing layer including aluminum carbideis formed between the aluminum and the carbon-containing layer, thecarbon-containing layer being formed on a surface of the aluminum.

SUMMARY

One aspect of an electrolytic capacitor according to the presentdisclosure includes: a capacitor element; and an electrolyte solution.The capacitor element includes: an anode foil on which a dielectriclayer is formed; a cathode foil which is opposite to the anode foil; anda conductive polymer layer that is interposed between the anode foil andthe cathode foil, the conductive polymer layer including a conductivepolymer. A conductive layer provided with a carbon layer includingconductive carbon is formed on the cathode foil. The conductive polymerlayer is a layer formed with use of a dispersion or a solutioncontaining the conductive polymer. And a proportion of water (watercontent) in the electrolyte solution ranges from 0.1% by mass to 6.0% bymass, inclusive.

According to the aspect of the present disclosure, it is possible tosecure a high capacitance, suppress an increase in ESR, and reduce theleakage current in the electrolytic capacitor including the electrolytesolution.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view illustrating an electrolyticcapacitor according to one exemplary embodiment of the presentdisclosure.

FIG. 2 is a schematic view illustrating a configuration of a capacitorelement of the electrolytic capacitor in FIG. 1.

DESCRIPTION OF EMBODIMENTS

Prior to describing exemplary embodiments of the present disclosure,problems in a conventional electrolytic capacitor are described.

In the solid electrolytic capacitor of Unexamined Japanese PatentPublication No. 2006-100478, the carbon-containing layer is provided inthe cathode from the viewpoint of increasing a capacitance, and furtherdecrease in ESR is expected from improvement in conductivity of thecathode. On the other hand, in the electrolytic capacitor including theelectrolyte solution as in Unexamined Japanese Patent Publication No.2008-010657, a defect of the dielectric layer formed on the anode can beeasily restored so that an increase in a leakage current and a decreasein withstand voltage can be suppressed. However, when a capacitorelement in which the carbon-containing layer is formed in the cathode isactually combined with the electrolyte solution, sufficient restoringproperty is not obtained in some cases. And sufficient adhesion of thecarbon-containing layer is unlikely to be secured, which sometimescauses increase of the ESR.

The present disclosure provides an electrolytic capacitor including anelectrolyte solution, which can secure a high capacitance and reduce theleakage current while suppressing an increase in ESR.

Hereinafter, exemplary embodiments of an electrolytic capacitoraccording to the present disclosure are described with appropriatereference to drawings. The exemplary embodiments below, however, are notto limit the present disclosure.

<<Electrolytic Capacitor>>

FIG. 1 is a schematic sectional view illustrating an electrolyticcapacitor according to one exemplary embodiment of the presentdisclosure. FIG. 2 is a schematic view illustrating a partiallydeveloped capacitor element included in the electrolytic capacitor.

In FIG. 1, the electrolytic capacitor includes capacitor element 10 andcapacitor element 10 is housed in an outer case (specifically, bottomedcase 11) together with an electrolyte solution (not shown). The outercase includes bottomed case 11 in which capacitor element 10 is housed,insulating sealing member 12 that seals an opening of bottomed case 11,and base plate 13 that covers sealing member 12. Bottomed case 11 is, ata part near an opening end, processed inward by drawing, and is, at theopening end, curled to swage sealing member 12.

As illustrated in FIG. 2, capacitor element 10 includes anode foil 21connected to lead tab 15A, cathode foil 22 connected to lead tab 15B,and separator 23. Anode foil 21 and cathode foil 22 are wound withseparator 23 interposed between the anode foil and the cathode foil. Thewound anode foil, cathode foil, and separator are also referred to as awound body. An outermost periphery of capacitor element 10 is fixed withfastening tape 24. FIG. 2 shows partially developed capacitor element 10before the outermost periphery of the capacitor element is fixed.

In capacitor element 10, anode foil 21 is a metal foil whose surface isroughened so as to have projections and recesses, and a dielectric layeris formed on the metal foil having the projections and recesses. Cathodefoil 22 opposite to anode foil 21 is a metal foil, and a conductivelayer is formed on the metal foil. The conductive layer formed oncathode foil 22 includes a carbon layer including conductive carbon. Aconductive polymer is attached to at least a part of a surface of thedielectric layer disposed on anode foil 21 and at least a part of asurface of the conductive layer disposed on cathode foil 22 so that aconductive polymer layer is formed. The conductive polymer may beattached to any position between anode foil 21 and cathode foil 22. Forexample, the conductive polymer covers at least a part of the surface ofthe dielectric layer formed on anode foil 21 and at least a part of thesurface of the conductive layer on cathode foil 22, and may furthercover at least a part of a surface of separator 23. As described above,the conductive polymer layer is interposed between anode foil 21 andcathode foil 22.

In the electrolytic capacitor, the conductive polymer (specifically, afilm including the conductive polymer) that covers at least a part ofthe surfaces of, for example, the anode foil, the cathode foil, and theseparator is generally referred to as a solid electrolyte layer (or aconductive polymer layer) in some cases.

Hereinafter, a configuration of the electrolytic capacitor according tothe exemplary embodiment of the present disclosure is described in moredetail.

A capacitor element includes an anode foil on which a dielectric layeris formed, a cathode foil on which a conductive layer is formed, and aconductive polymer layer interposed between the anode foil and thecathode foil. The capacitor element may further include a separator asnecessary.

(Capacitor Element) (Anode Foil)

Examples of the anode foil include a metal foil whose surface isroughened. A type of the metal that constitutes the metal foil is notparticularly limited, but it is preferred to use a valve metal such asaluminum, tantalum, or niobium, or an alloy including a valve metal,from the viewpoint of facilitating formation of the dielectric layer.

Roughening the surface of the metal foil can be performed by a publiclyknown method. By the roughening, a plurality of projections and recessesare formed on the surface of the metal foil. The roughening ispreferably performed by subjecting the metal foil to an etchingtreatment, for example. The etching treatment may be performed by, forexample, a DC electrolytic method or an AC electrolytic method.

(Dielectric Layer)

The dielectric layer is formed on a surface of the anode foil.Specifically, the dielectric layer is formed on a roughened surface ofthe metal foil, so that the dielectric layer is formed along an innerwall surface of pores and pits on the surface of the anode foil.

A method for forming the dielectric layer is not particularly limited,and the dielectric layer can be formed by subjecting the metal foil toan anodizing treatment. The anodizing treatment may be performed by, forexample, immersing the metal foil in an anodizing solution such as anammonium adipate solution. In the anodizing treatment, a voltage may beapplied in a state in which the metal foil is immersed in the anodizingsolution, as necessary.

Normally, a large metal foil formed of, for example, a valve metal issubjected to the roughening treatment and the anodizing treatment fromthe viewpoint of mass productivity. In this case, the treated foil iscut into a desired size to arrange anode foil 21 on which the dielectriclayer is formed.

(Cathode Foil)

Also for cathode foil 22, a metal foil is used as in the anode foil. Atype of the metal is not particularly limited, but it is preferred touse a valve metal such as aluminum, tantalum, or niobium, or an alloyincluding a valve metal.

When the conductive polymer layer is formed on a surface of the cathodefoil with use of a dispersion or a solution containing a conductivepolymer, it is possible to obtain a conductive polymer layer having highhomogeneity and high flexibility.

(Conductive Layer)

The conductive layer is preferably, as a whole layer, formed of aninorganic material (e.g., a metal, a metal compound, and/or conductivecarbon) having conductivity and is distinguished from the conductivepolymer layer formed of an organic material. The conductive layer canalso be referred to as an inorganic conductive layer because theconductive layer is, as a whole layer, formed of an inorganic material.The conductive layer includes at least a carbon layer in contact withthe conductive polymer layer.

The carbon layer includes conductive carbon. Examples of the conductivecarbon include amorphous carbon, carbon black such as acetylene black,soft carbon, hard carbon, graphite, and a carbon fiber such as a carbonnanotube. The carbon layer may include one of these conductive carbonmaterials or two or more of these conductive carbon materials.

The carbon layer may be a layer including the conductive carbon and abinder, but a ratio of the conductive carbon is preferred to be as highas possible. A ratio of the conductive carbon in the carbon layer ispreferably 95% by mass or more or 99% by mass or more, for example. Thecarbon layer may be formed by removing a binder by a heat treatment froma layer including the conductive carbon and the binder. Particularly,the carbon layer is preferably a layer formed of the conductive carbon.Especially, the carbon layer is preferably a deposition film of theconductive carbon (particularly, amorphous carbon).

The conductive layer may include only the carbon layer, and may furtherinclude a base layer having conductivity from the viewpoint ofincreasing the adhesion. The base layer is formed on a surface of thecathode foil, and the carbon layer can be formed on the base layer. Whenthe conductive layer includes the base layer, the carbon layer may bedirectly formed on a surface of the base layer or may be formed on thebase layer with another conductive layer interposed between the baselayer and the carbon layer.

The base layer that constitutes a part of the conductive layer includesa non-carbonaceous inorganic material having conductivity such as ametal or a conductive metal compound. Examples of the metal includetitanium and/or nickel. As the metal compound, a metal nitride such astitanium nitride is preferable.

As in Unexamined Japanese Patent Publication No. 2006-100478, when theconductive layer formed on the cathode foil includes a carbide componentsuch as aluminum carbide, lifetime characteristics are sometimesdecreased. For example, conductive aluminum carbide reacts with water inthe electrolytic capacitor (particularly, in the electrolyte solution)to generate methane and insulating aluminum hydroxide. And then theinsulating aluminum hydroxide comes to exist between the cathode foiland the conductive polymer layer. The existence of the insulatingaluminum hydroxide consequently makes difficult securement of electricalconnection between the cathode foil and the conductive polymer layerwhen charging and discharging are repeated. Therefore, from theviewpoint of increasing the lifetime characteristics, the conductivelayer is preferred not to substantially include a carbide component(e.g., aluminum carbide). The carbide component (e.g., aluminum carbide)in the conductive layer is preferably 1% by mass or less.

A thickness of the conductive layer ranges, for example, from 1 nm to 10μm, inclusive. When the carbon layer is a deposition film, the thicknessof the conductive layer may range, for example, from 1 nm to 100 nm,inclusive. When the carbon layer is formed of a layer including theconductive inorganic material and a binder, the thickness of theconductive layer may range, for example, from 100 nm to 10 μm,inclusive. The conductive layer having a thickness in such rangesfacilitates securement of high conductivity and a high capacitance.

The thickness of the conductive layer may be an average thicknessobtained by averaging thicknesses measured at a plurality of points(e.g., 10 points) in a sectional image.

(Separator)

As separator 23, for example, a nonwoven fabric may be used thatincludes a fiber of, for example, cellulose, polyethylene terephthalate,a vinylon, or a polyamide (e.g., an aliphatic polyamide and an aromaticpolyamide such as aramid).

Capacitor element 10 can be manufactured by a publicly known method. Forexample, capacitor element 10 may be manufactured by stacking anode foil21 on which the dielectric layer is formed and cathode foil 22 on whichthe conductive layer is formed, with separator 23 interposed between theanode foil and the cathode foil, and then forming the conductive polymerlayer between anode foil 21 and cathode foil 22. Capacitor element 10may also be manufactured by winding anode foil 21 on which thedielectric layer is formed and cathode foil 22 on which the conductivelayer is formed, with separator 23 interposed between the anode foil andthe cathode foil, to form a wound body as illustrated in FIG. 2, andforming the conductive polymer layer between anode foil 21 and cathodefoil 22. When the wound body is formed, the winding may be performedwhile lead tabs 15A, 15B are rolled in the anode foil, the cathode foil,and the separator, to cause lead tabs 15A, 15B to stand up from thewound body as illustrated in FIG. 2.

A material for lead tabs 15A, 15B is not particularly limited as long asthe material is a conductive material. Surfaces of lead tabs 15A, 15Bmay be subjected to an anodizing treatment. Further, lead tabs 15A, 15Bmay be covered with a resin material at a part in contact with sealingmember 12 and a part connected to lead wires 14A, 14B.

A material for lead wires 14A, 14B connected to lead tabs 15A, 15B,respectively, is not also particularly limited, and for example, aconductive material may be used.

An end of an outer surface of anode foil 21, cathode foil 22 orseparator 23 that is positioned at an outermost layer of the wound body(cathode foil 22 in FIG. 2) is fixed with fastening tape 24. When anodefoil 21 is arranged by cutting a large metal foil, the capacitor elementin a state of, for example, the wound body, may further be subjected toan anodizing treatment in order to provide a dielectric layer on acutting surface of anode foil 21.

(Conductive Polymer Layer)

The conductive polymer layer is interposed between anode foil 21 andcathode foil 22. The conductive polymer layer is preferably formed on atleast a part of a surface of the dielectric layer formed on the surfaceof anode foil 21, so as to cover the dielectric layer. The conductivepolymer layer is more preferably formed so as to cover as large a regionof the dielectric layer as possible. The conductive polymer layer ispreferably formed on at least a part of a surface of the conductivelayer formed on the surface of cathode foil 22, so as to cover theconductive layer. The conductive polymer layer is more preferably formedso as to cover as large a region of the conductive layer as possible.When the capacitor element includes the separator, the conductivepolymer layer may be formed on not only the surfaces of the dielectriclayer and the conductive layer but also a surface of the separator.

Generally, a method for providing the conductive polymer layer isclassified to two cases. One case is forming the conductive polymerlayer with use of a dispersion obtained by dispersing fine particles ofa conductive polymer in a dispersion medium or a solution obtained bydissolving a conductive polymer in a solvent. And the other case isforming the conductive polymer layer by polymerizing a precursor of aconductive polymer (e.g., a monomer or an oligomer that is to be a rawmaterial of the conductive polymer) while the precursor is in contactwith the anode foil and the cathode foil.

In the latter case, due to strong reactivity of an oxidant for apolymerization reaction or a monomer itself, the cathode foil and theanode foil corrode to consequently deteriorate contact between the foilsand the conductive polymer layer as well as contact between the cathodefoil and the conductive layer, and thus decreasing the capacitance maydecreases or the ESR increases. An oxidant and a monomer that remainafter the polymerization are not sufficiently taken away even by washingto adversely affect a lifetime of the electrolytic capacitor.

In the present disclosure, the conductive polymer layer is formed withuse of a dispersion obtained by dispersing fine particles of aconductive polymer in a dispersion medium, or a solution obtained bydissolving a conductive polymer in a solvent. Such a conductive polymerlayer is formed by attachment of the conductive polymer to a peripheryof the anode foil and the cathode foil through contacting the dispersionor the solution to the anode foil and the cathode foil. By using such aconductive polymer layer, can suppress corrosion of the cathode foil andthe anode foil can be suppressed so that the contact of the anode foiland the conductive layer of the cathode foil with the conductive polymerlayer can be secured, and further the contact between the cathode foiland the conductive layer can be secured. Accordingly, an increase in ESRcan be suppressed, and a high capacitance can easily be secured.

(Conductive Polymer)

Examples of the conductive polymer included in the conductive polymerlayer include polypyrrole, polythiophene, polyfuran, polyaniline,polyacetylene, polyphenylene, polyphenylene vinylene, polyacene, andpolythiophene vinylene. These conductive polymers may be used alone orin combination of two or more conductive polymers, or may be a copolymerof two or more monomers.

In the present specification, polypyrrole, polythiophene, polyfuran,polyaniline, and the like mean polymers having, as a basic skeleton,polypyrrole, polythiophene, polyfuran, polyaniline, and the like,respectively. Therefore, polypyrrole, polythiophene, polyfuran,polyaniline, and the like also include derivatives of polypyrrole,polythiophene, polyfuran, polyaniline, and the like, respectively. Forexample, polythiophene includes poly(3,4-ethylenedioxythiophene) and thelike.

These conductive polymers may be used alone or in combination of two ormore conductive polymers.

A weight average molecular weight of the conductive polymer is notparticularly limited and ranges, for example, from 1,000 to 1,000,000,inclusive.

(Dopant)

The conductive polymer layer may include a dopant. The dopant may beincluded in the conductive polymer layer while doped in the conductivepolymer, or may be included in the conductive polymer layer while boundwith the conductive polymer.

As the dopant, a polyanion can be used. Specific examples of thepolyanion include polyanions such as polyvinylsulfonic acid,polystyrenesulfonic acid, polyallylsulfonic acid, polyacrylsulfonicacid, polymethacrylsulfonic acid,poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprenesulfonicacid, and polyacrylic acid. Especially, a polyanion derived frompolystyrenesulfonic acid is preferable. These polyanions may be usedalone or in combination of two or more polyanions. Further, thesepolyanions may be a polymer of a single monomer or a copolymer of two ormore monomers.

A weight average molecular weight of the polyanion is not particularlylimited and ranges, for example, from 1,000 to 1,000,000, inclusive. Theconductive polymer including such a polyanion is easily andhomogeneously dispersed in a solvent and is easily and uniformlyattached to the surfaces of the dielectric layer and the conductivelayer.

(Electrolyte Solution)

In a solid electrolytic capacitor, by using an electrolyte solution, aproperty of restoring the dielectric layer is expected to increase tosuppress the leakage current. However, when the carbon layer is actuallycombined with the electrolyte solution, sufficient property of restoringthe dielectric layer cannot be obtained in some cases.

On the other hand, even when in an actual combination of the cathodefoil on which the conductive layer including the carbon layer is formed,is actually combined with the electrolyte solution, by using theelectrolyte solution having a proportion of water of 0.1% by mass ormore, an effect of restoring the dielectric layer can be obtained tothus reduce the leakage current.

In the meantime, the electrolyte solution containing water decreases thecapacitance or increases the ESR along with lapse of using time of theelectrolytic capacitor. These phenomena are considered to be caused by adecrease in adhesion between the cathode foil and the conductive layerincluding the carbon layer due to the water in the electrolyte solution.Further, in a case of using the electrolyte solution containing waterfor an electrolytic capacitor having the conductive polymer layer formedby polymerizing a precursor of the conductive polymer while theprecursor is in contact with the cathode foil, remained oxidant andprecursor become active by an action of the water in the electrolytesolution, so that the foils corrode to decrease the adhesion of thecathode foil to the conductive layer, and the adhesion of the anode foiland the conductive layer to the conductive polymer layer.

In the present exemplary embodiment, the conductive polymer layer isformed with use of the dispersion or the solution containing theconductive polymer, and a proportion of the water in the electrolytesolution is set to 6.0% by mass or less, and thus a decrease in theadhesion of the cathode foil to the conductive layer, and the adhesionof the anode foil and the conductive layer to the conductive polymerlayer can be suppressed. Accordingly, an increase in ESR can besuppressed while a high capacitance is secured.

The electrolyte solution having a water of less than 0.1% by mass isinferior in the property of restoring the dielectric layer, so that theleakage current cannot be suppressed. Whereas the electrolyte solutionhaving a water of more than 6.0% by mass increases the ESR. As describedabove, it is important to set a proportion of the water in theelectrolyte solution to a range from 0.1% by mass to 6.0% by mass,inclusive, in the present exemplary embodiment. From the viewpoint offurther reducing the ESR, a proportion of the water in the electrolytesolution is preferably 5.0% by mass or less. From the viewpoint offurther reducing the leakage current, a proportion of the water in theelectrolyte solution is preferably 0.5% by mass or more, more preferably1.0% by mass or more. Any of these minimum and maximum values can becombined. A proportion of the water in the electrolyte solution mayrange from 0.1% by mass to 5.0% by mass, inclusive, or from 0.5% by massto 5.0% by mass, inclusive, for example.

The water in the electrolyte solution is not necessarily containedinitially in the electrolyte solution used to assemble the electrolyticcapacitor, but the water may be mixed in the electrolyte solution in aprocess of assembling the electrolytic capacitor. For example, the watermay be included in a constituent of the electrolytic capacitor inadvance, or may be included in the dispersion liquid or the solutioncontaining the conductive polymer.

The electrolyte solution is not particularly limited as long as theelectrolyte solution has the amount of water described above, and anonaqueous solvent may be used, or a solution may also be used thatcontains a nonaqueous solvent and an ionic substance (solute) dissolvedin the nonaqueous solvent. The nonaqueous solvent is a collective termfor liquids other than water and liquids containing water, and includesan organic solvent and an ionic liquid.

Examples of the nonaqueous solvent contained in the electrolyte solutioninclude a polyol (e.g., alkylene glycols such as ethylene glycol andpropylene glycol; polyalkylene glycols such as polyethylene glycol; andglycerins such as glycerin and polyglycerin), cyclic sulfones such assulfolane, lactones such as γ-butyrolactone (γBL), amides such asN-methylacetamide, N,N-dimethylformamide, and N-methyl-2-pyrrolidone,esters such as methyl acetate, ethers such as 1,4-dioxane, ketones suchas methyl ethyl ketone, and formaldehyde. A single one or two or more incombination of the nonaqueous solvents may be used.

The electrolyte solution preferably contains at least a solvent (firstsolvent) having no boiling point or having a high boiling point (e.g.,180° C. or more) among the nonaqueous solvents described above. Theelectrolyte solution containing the first solvent can suppress depletionof the electrolyte solution even when the electrolytic capacitor is usedfor a long period, so that it is possible to secure high reliabilityover a long period.

The boiling point of the first solvent should be 180° C. or more and maybe 200° C. or more. As the first solvent, a polyol is preferable. Forexample, polyethylene glycol and polyglycerin sometimes do not have aboiling point depending on molecular weights of polyethylene glycol andpolyglycerin. Such a compound (limited to a liquid, however) is alsopreferable as the first solvent.

In the meantime, the first solvent is not necessarily contained in theelectrolyte solution used to assemble the electrolytic capacitor, butthe first solvent may be contained in a treatment solution used in aprocess of assembling the electrolytic capacitor. For example, thedispersion liquid or the solution containing the conductive polymer maycontain the first solvent. The first solvent having no boiling point orhaving a high boiling point remains in the electrolytic capacitorassembled. The first solvent that has remained oozes into theelectrolyte solution housed in the electrolytic capacitor, so that theelectrolyte solution in the electrolytic capacitor comes to contain thefirst solvent. From the viewpoint of easily securing the adhesionbetween the conductive polymer layer and the conductive layer, aproportion of the first solvent contained in the dispersion ispreferably 50% by mass or less of the dispersion or the solution.

The proportion of the first solvent contained in the electrolytesolution preferably ranges from 3% by mass to 90% by mass, inclusive.The electrolyte solution having a proportion of the first solvent insuch a range can suppress a decrease in the adhesion between theconductive polymer layer and the conductive layer, and further improve afunction of restoring the dielectric layer.

As the solute contained in the electrolyte solution, there can beexemplified a salt of an anion and a cation, and an organic salt ispreferable, in which at least one of the anion and the cation is anorganic substance. Examples of the organic salt include trimethylaminemaleate, triethylamine borodisalicylate, ethyldimethylamine phthalate,mono 1,2,3,4-tetramethylimidazolinium phthalate, and mono1,3-dimethyl-2-ethylimidazolinium phthalate. A single one or two or morein combination of the solutes may be used.

<<Method for Producing Electrolytic Capacitor>>

Hereinafter, one example of a method for producing the electrolyticcapacitor according to the exemplary embodiment of the presentdisclosure is described according to each of steps.

The electrolytic capacitor can be obtained through the steps ofpreparing a dispersion or a solution (first treatment solution)containing a conductive polymer (first step); arranging an anode foil onwhich a dielectric layer is formed (second step); arranging a cathodefoil on which a conductive layer is formed (third step); obtaining acapacitor element by impregnating with the first treatment solution theanode foil, the cathode foil, and a separator interposed between theanode foil and the cathode foil, as necessary (fourth step); andimpregnating the capacitor element with an electrolyte solution (fifthstep). A conductive polymer layer can be formed through the fourth step.A solvent component may be removed in an appropriate stage.

(i) First Step

In the first step, a first treatment solution is prepared that containsa conductive polymer (and a dopant) and a solvent (second solvent).

The first treatment solution can be obtained by, for example, dispersingor dissolving the conductive polymer (and the dopant) in the secondsolvent. Alternatively, the first treatment solution can also beobtained by, for example, polymerizing in the second solvent a rawmaterial of the conductive polymer (e.g., a precursor such as a monomerand/or an oligomer of the conductive polymer) in presence of the dopant.In the case of preparing the first treatment solution throughpolymerization, an unreacted raw material and a byproduct may be removedas necessary. Alternatively, polymerization may be performed with use ofa part of the second solvent to give a mixture to which the remainingpart of the second solvent is added.

The second solvent is not particularly limited, and may be water or anonaqueous solvent (e.g., an organic solvent and an ionic liquid).Especially, the second solvent is preferably a polar solvent. The polarsolvent may be a protic solvent or an aprotic solvent.

Examples of the protic solvent include a monohydric alcohol (e.g.,methanol, ethanol, propanol, and butanol); a polyol (e.g., alkyleneglycols such as ethylene glycol and propylene glycol, polyalkyleneglycols such as polyethylene glycol, and glycerins such as glycerin andpolyglycerin); glycol monoethers such as diethylene glycol monobutylether; formaldehyde; and water.

Examples of the aprotic solvent include amides such asN-methylacetamide, N,N-dimethylformamide, and N-methyl-2-pyrrolidone;esters such as methyl acetate; ketones such as methyl ethyl ketone andγ-butyrolactone; ethers (cyclic ethers) such as 1,4-dioxane; sulfonessuch as dimethyl sulfoxide and sulfolane; and carbonate compounds (e.g.,cyclic carbonates) such as propylene carbonate.

Especially, the second solvent is preferably a protic solvent. From theviewpoint of increasing handleability of the first treatment solutionand dispersibility of the conductive polymer, the second solventpreferably contains water. The second solvent containing a polyol islikely to increase the conductivity of the conductive polymer layer (inother words, likely to further decrease the ESR). Accordingly, thesecond solvent containing a polyol is also preferable, and use of thesecond solvent is also preferable that contains at least water and apolyol.

The first treatment solution is preferably a dispersion obtained bydispersing the conductive polymer (and the dopant) in the secondsolvent. In the dispersion, the conductive polymer and/or the dopant ispreferred to be particles (or a powder). An average particle size of theparticles dispersed in the dispersion preferably ranges from 5 nm to 100nm, inclusive. The average particle size can be determined, for example,from a particle size distribution obtained by a dynamic light scatteringmethod.

A ratio of the dopant contained in the first treatment solutionpreferably ranges from 10 parts by mass to 1,000 parts by mass,inclusive, more preferably from 50 parts by mass to 200 parts by mass,inclusive, relative to 100 parts by mass of the conductive polymer.

A concentration of the conductive polymer (including the dopant or apolyanion) in the first treatment solution preferably ranges, forexample, from 0.5% by mass to 3% by mass, inclusive. The first treatmentsolution having such a concentration of the conductive polymer issuitable for attachment of an appropriate amount of the conductivepolymer and is easily impregnated to also give advantages forproductivity improvement.

The first treatment solution may contain, for example, a publicly knownadditive as necessary.

(ii) Second Step

In the second step, a surface of an anode foil is subjected to, forexample, an anodizing treatment to form a dielectric layer on thesurface of the anode foil, as described above.

(iii) Third Step

In the third step, a cathode foil is arranged on a surface of which aconductive layer including a carbon layer is formed. More specifically,the carbon layer including conductive carbon can be formed on thecathode foil to form the conductive layer, for example.

The carbon layer may be formed by attaching powder of conductive carbonto the surface of the cathode foil. Alternatively, the carbon layer mayalso be formed by forming a coated film by coating the surface of thecathode foil with a mixture (e.g., a slurry) containing the conductivecarbon and a binder, and drying the coated film or removing the binderby subjecting the coated film to a heat treatment.

The conductive layer including the carbon layer is preferred to beformed by depositing the conductive carbon on the surface of the cathodefoil by a gas phase method. Examples of the gas phase method includechemical vapor deposition, vacuum vapor deposition, sputtering, and ionplating.

In the third step, the conductive layer may be formed by forming a baselayer on the surface of the cathode foil as necessary and forming, asdescribed above, the carbon layer on the base layer. The base layerconstituting the conductive layer can be formed, in the same manner asin the carbon layer, with use of a non-carbonaceous inorganic materialhaving conductivity. The base layer is preferred to be formed bydepositing the non-carbonaceous inorganic material having conductivityon the surface of the cathode foil by the gas phase method.

(iv) Fourth Step

In the fourth step, the first treatment solution is impregnated into theanode foil on which the dielectric layer is formed, the cathode foil onwhich the conductive layer is formed, and a separator as necessary. Morespecifically, in the fourth step, the first treatment solution may beimpregnated into a wound body obtained by winding the anode foil onwhich the dielectric layer is formed and the cathode foil on which theconductive layer is formed, with the separator interposed between theanode foil and the cathode foil. The impregnation with the firsttreatment solution may be performed by immersing the wound body in thefirst treatment solution or injecting the first treatment solution intothe wound body.

The impregnation with the first treatment solution may be performedunder atmospheric pressure, but may also be performed under a reducedpressure, in an atmosphere ranging, for example, from 10 kPa to 100 kPa,inclusive, preferably from 40 kPa to 100 kPa, inclusive. Theimpregnation may also be performed under ultrasonic vibration asnecessary. An impregnation period depends on a size of capacitor element10, and ranges, for example, from 1 second to 5 hours, inclusive,preferably from 1 minute to 30 minutes, inclusive.

The anode foil and the cathode foil (and further the separator) may bedried as necessary after impregnated with the first treatment solution.The drying removes at least a part of the second solvent. The drying maybe performed by heating, and may also be performed under a reducedpressure as necessary.

As described above, the conductive polymer layer is formed between theanode foil and the cathode foil through the fourth step to thus formcapacitor element 10.

(v) Fifth Step

In the fifth step, the capacitor element obtained in the fourth step isimpregnated with an electrolyte solution.

The impregnation of capacitor element 10 with the electrolyte solutionis not particularly limited and can be performed by a publicly knownmethod. For example, capacitor element 10 may be immersed in theelectrolyte solution, or the electrolyte solution may be injected into acontainer housing capacitor element 10. The impregnation of capacitorelement 10 with the electrolyte solution may be performed under areduced pressure (e.g., 10 kPa to 100 kPa) as necessary.

A proportion of water in the electrolyte solution used is preferred tobe appropriately adjusted so that the proportion of water contained inthe electrolyte solution after the electrolytic capacitor is assembledis set to the ranges described above.

(Others)

Capacitor element 10 may be encapsulated. More specifically, first,capacitor element 10 is housed in bottomed case 11 so that lead wires14A, 14B are positioned on an open upper surface of bottomed case 11. Asa material for bottomed case 11, there can be used metals such asaluminum, stainless steel, copper, iron and brass, or alloys of thesemetals.

Next, sealing member 12 formed so as to allow lead wires 14A, 14B topenetrate the sealing member is disposed above capacitor element 10 toencapsulate capacitor element 10 in bottomed case 11. Sealing member 12is sufficient as long as the sealing member is an insulating substance.As the insulating substance, an elastic body is preferable, andespecially preferred is, for example, a high heat resistance siliconerubber, fluororubber, ethylene propylene rubber, chlorosulfonatedpolyethylene rubber (e.g., Hypalon rubber), butyl rubber or isoprenerubber.

Next, bottomed case 11 is, at a part near an opening end, processed bytransverse drawing, and is, at the opening end, curled to swage sealingmember 12. Then, base plate 13 is disposed on a curled part of thebottomed case to complete the electrolytic capacitor as illustrated inFIG. 1. Then, an aging treatment may be performed while a rated voltageis applied.

In the exemplary embodiments described above, a wound electrolyticcapacitor has been described. The application range of the presentdisclosure, however, is not limited to the wound electrolytic capacitorand can also be applied to other electrolytic capacitors such as a chipelectrolytic capacitor including a metal sintered body in place of theanode foil, and a laminated electrolytic capacitor including a metalplate in place of the anode foil.

EXAMPLES

Hereinafter, the present disclosure is specifically described withreference to examples and comparative examples. The present disclosure,however, is not limited to the examples below.

Example 1

A wound electrolytic capacitor having a rated voltage of 35 V and arated electrostatic capacity of 47 μF, as illustrated in FIG. 1, wasmanufactured in the following procedure, and evaluation for theelectrolytic capacitor was conducted.

(1) Production of Electrolytic Capacitor (Preparation of Anode FoilHaving Dielectric Layer)

A 100-μm-thick aluminum foil was subjected to an etching treatment toroughen a surface of the aluminum foil. Then, a dielectric layer wasformed on the surface of the aluminum foil by an anodizing treatmentwith an ammonium adipate aqueous solution to arrange an anode foilhaving the dielectric layer.

(Preparation of Cathode Foil Having Conductive Layer)

A cathode foil was arranged by forming a conductive layer on a surfaceof a 30-μm-thick aluminum foil through ion plating of conductive carbon.A thickness of the conductive layer was 3 nm.

(Manufacturing of Wound Body)

An anode lead tab and a cathode lead tab were connected to the anodefoil and the cathode foil, respectively, and the anode foil and thecathode foil were would with a separator interposed between the anodefoil and the cathode foil while the lead tabs were rolled in the anodefoil, the cathode foil and the separator, to give a wound body. Ends ofthe lead tabs protruding from the wound body were connected to an anodelead wire and a cathode lead wire, respectively. Then, the manufacturedwound body was subjected to an anodizing treatment again to form adielectric layer at a cut end of the anode foil. Next, an end of anouter surface of the wound body was fixed with a fastening tape.

(Preparation of First Treatment Solution)

A mixed solution was prepared by dissolving 3,4-ethylenedioxythiopheneand a dopant, i.e., polystyrenesulfonic acid in ion-exchanged water.Ferric sulfate and sodium persulfate (an oxidant) dissolved inion-exchanged water were added to the resultant solution while thesolution was stirred, to cause a polymerization reaction. After thereaction, a resultant reaction solution was dialyzed to remove unreactedmonomers and an excessive oxidant, so that a dispersion liquid wasobtained that included poly(3,4-ethylene dioxythiophene) doped withpolystyrenesulfonic acid (PEDOT-PSS). A concentration of PEDOT-PSS inthe dispersion liquid was about 2% by mass, and a mass ratio between PSSand PEDOT (=PSS:PEDOT) was about 2:1. Ethylene glycol (second solvent)at 5% by mass was added to the resultant dispersion liquid and stirredto prepare a first treatment solution having a state of a dispersionliquid.

(Impregnation with First Treatment Solution)

The wound body was impregnated with the first treatment solution for 5minutes. Next, the wound body was heated at 150° C. for 20 minutes toremove a solvent component. Thus, a capacitor element was manufacturedin which a conductive polymer layer was formed between the anode foiland the cathode foil.

(Impregnation with Electrolyte Solution)

Next, the capacitor element was impregnated with an electrolyte solutionunder a reduced pressure. Used as the electrolyte solution was asolution containing γBL, glycerin, and mono(ethyldimethylamine)phthalate (solute) at a mass ratio of 50:25:25. A proportion of watercontained in γBL and glycerin used was measured in advance, and theproportion of water in the electrolyte solution was adjusted by addingwater to the electrolyte solution or evaporating water by warming sothat the water in the electrolyte solution becomes an intendedproportion of water. In the electrolyte solution, γBL and glycerin are afirst solvent.

(Encapsulation of Capacitor Element)

The electrolyte solution-impregnated capacitor element was housed in anouter case as illustrated in FIG. 1 and encapsulated to manufacture anelectrolytic capacitor. A total of 300 electrolytic capacitors weremanufactured in the same manner.

(2) Evaluation

Evaluation below was conducted for randomly selected 120 electrolyticcapacitors, and an average value was calculated.

(a) Proportion of Water

The electrolyte solution was extracted from the electrolytic capacitorassembled, and the proportion of water (% by mass) in the electrolytesolution was measured by a Karl Fisher method. The measurement resultindicated that the proportion of water in the electrolyte solution was0.10% by mass.

(b) Proportion of First Solvent in Electrolyte Solution

The electrolyte solution was extracted from the electrolytic capacitor,and a proportion (% by mass) of the first solvent contained in theelectrolyte solution was measured by gas chromatography. The measurementresult indicated that the proportion of the first solvent in theelectrolyte solution was 75.0% by mass.

(c) Electrostatic Capacity and ESR Value

An electrostatic capacity (μF) and an ESR value (mΩ) were measured asinitial characteristics of the electrolytic capacitor. Specifically, aninitial electrostatic capacity (μF) at a frequency of 120 Hz wasmeasured for the electrolytic capacitor with an LCR meter for 4-terminalmeasurement. In addition, an ESR value (mΩ) at a frequency of 100 kHzwas measured for the electrolytic capacitor with an LCR meter for4-terminal measurement.

Also measured in the same manner as in the initial characteristicsdescribed above were an electrostatic capacity (μF) and an ESR value(mΩ) after a test of leaving the electrolytic capacitor to stand at ahigh temperature of 125° C. for 4000 hours.

The electrostatic capacities and the ESR values were measured for eachrandomly selected 120 electrolytic capacitors, and average values forthe electrostatic capacities and the ESR values were calculated.

(d) Leakage Current (LC)

As the initial characteristics of the electrolytic capacitor, the ratedvoltage was applied to the electrolytic capacitor and the leakagecurrent (μA) at 2 minutes after the application was measured.

Also measured in the same manner as in the initial characteristicsdescribed above was the leakage current (μA) after the test of leavingthe electrolytic capacitor to stand at a high temperature of 125° C. for4000 hours.

Examples 2 to 6 and Comparative Examples 1 to 2

An electrolytic capacitor was manufactured in the same manner as inExample 1 except for adjusting the proportion of water in theelectrolyte solution so that the proportion of water in the electrolytesolution of the electrolytic capacitor assembled gave the valueindicated in Table 1, and the evaluation was conducted. The proportionof the first solvent in the electrolyte solution of the electrolyticcapacitor assembled was as indicated below.

Example 2: 76.0% by mass

Example 3: 75.8% by mass

Example 4: 75.6% by mass

Example 5: 75.7% by mass

Example 6: 75.6% by mass

Comparative Example 1: 75.8% by mass

Comparative Example 2: 75.1% by mass

Comparative Example 3

A solution was prepared by mixing 1 part by mass of3,4-ethylenedioxythiophene as a polymerizable monomer, 2 parts by massof ferric p-toluenesulfonate that served as an oxidant and a dopantcomponent, and 4 parts by mass of n-butanol as a solvent. A wound bodymanufactured in the same manner as in Example 1 was immersed in theresultant solution, picked up from the solution and left to stand at 85°C. for 60 minutes to manufacture a capacitor element in which theconductive polymer layer was formed between the anode foil and thecathode foil. An electrolytic capacitor was manufactured in the samemanner as in Example 3 except for using the resultant capacitor element,and the evaluation was conducted.

The proportion of the first solvent in the electrolyte solution of theelectrolytic capacitor was 74.4% by mass.

Comparative Example 4

In Comparative Example 4, an electrolytic capacitor was manufactured inthe same manner as in Example 3 except for not forming a conductivelayer on the surface of the aluminum foil, and the evaluation wasconducted.

Comparative Example 5

In Comparative Example 5, a solid electrolytic capacitor wasmanufactured that did not include an electrolyte solution. Morespecifically, in the same manner as in Example 1, a capacitor elementwas manufactured in which the conductive polymer layer was formedbetween the anode foil and the cathode foil. The resultant capacitorelement was housed in an outer case and encapsulated to give a solidelectrolytic capacitor, and the evaluation was conducted in the samemanner as in Example 1.

Table 1 shows results of the examples and the comparative examples. A1to A6 denote Examples 1 to 6, and B1 to B5 denote Comparative Examples 1to 5.

TABLE 1 Proportion Electrostatic ESR LC of water in capacity (μF) (mΩ)(μA) electrolyte After After After solution 4000 4000 4000 (% by mass)Initial hours Initial hours Initial hours B1 0.05 55.5 49.8 63.0 120.24.8 330.1 A1 0.10 55.5 49.6 63.0 100.0 4.8 15.8 A2 0.50 55.5 49.5 63.097.8 4.7 9.3 A3 1.00 55.5 49.5 63.0 98.2 4.7 7.9 A4 3.00 55.5 49.3 63.098.2 4.7 7.6 A5 5.00 55.5 49.1 65.0 100.3 4.6 7.2 A6 6.00 55.5 49.1 65.0122.7 4.5 7.2 B2 8.00 55.5 48.5 70.0 153.0 4.5 7.2 B3 1.00 55.5 49.572.0 255.6 4.7 8.1 B4 1.00 47.0 43.0 63.0 120.2 4.7 7.9 B5 — 55.5 49.963.0 130.1 4.8 484.4

As shown in Table 1, in the examples, a high electrostatic capacitycould be obtained in both the initial measurement and the measurementafter the electrolytic capacitor was left to stand at the hightemperature for 4000 hours. The initial ESR and leakage current weresuppressed low. In Comparative Example 1 where the proportion of waterin the electrolyte solution was 0.05% by mass, the effect of restoringthe dielectric layer could not sufficiently be obtained, and the leakagecurrent after the electrolytic capacitor was left to stand at the hightemperature remarkably increased. In Comparative Example 2 where theproportion of water in the electrolyte solution was 8.00% by mass and inComparative Example 3 where the conductive polymer layer was formed bypolymerization, the ESR after the electrolytic capacitor was left tostand at the high temperature was remarkably higher than in theexamples. In Comparative Example 4 where a conductive layer was notprovided, the initial electrostatic capacity was lower than in theexamples. In Comparative Example 5 where the solid electrolyticcapacitor was manufactured, the leakage current increased after thesolid electrolytic capacitor was left to stand for 4000 hours.

The present disclosure can be utilized for an electrolytic capacitorthat includes a capacitor element having a conductive polymer layer, andan electrolyte solution.

What is claimed is:
 1. A method for producing an electrolytic capacitor,the method comprising: forming a capacitor element; and impregnating thecapacitor element with a liquid containing a first solvent having noboiling point or a boiling point of 180° C. or more, wherein: theforming of the capacitor element includes: preparing an anode foil onwhich a dielectric layer is formed and a cathode foil provided with afirst layer on the cathode foil, the first layer including at least oneselected from the group consisting of carbon, nickel, a nickel compound,titanium, and a titanium compound; and forming a conductive polymerlayer including a conductive polymer between the anode foil and thecathode foil that are disposed to face each other, and a proportion ofwater in the liquid ranges from 0.1% by mass to 6.0% by mass, inclusive.2. The method according to claim 1, wherein the first layer includes abase layer, the base layer being in contact with the cathode foil. 3.The method according to claim 2, wherein the base layer includes atleast one of a metal and a metal compound.
 4. The method according toclaim 1, wherein the first layer includes a carbon layer, the carbonlayer including the carbon.
 5. The method according to claim 1, whereinthe first layer includes aluminum carbide.
 6. The method according toclaim 5, wherein a proportion of the aluminum carbide in the first layeris more than 0% by mass and 1% by mass or less.
 7. The method accordingto claim 1, wherein the base layer includes aluminum carbide.
 8. Themethod according to claim 1, wherein a thickness of the first layerranges from 1 nm to 10 μm, inclusive.
 9. The method according to claim1, wherein a thickness of the first layer ranges from 100 nm to 10 μm,inclusive.
 10. The method according to claim 1, wherein the forming ofthe conductive polymer layer includes forming the conductive polymerlayer by using a dispersion or a solution containing the conductivepolymer.
 11. The method according to claim 1, wherein the first solventcontains a polyol.
 12. The method according to claim 1, wherein aproportion of the first solvent contained in the liquid ranges from 3%by mass to 90% by mass, inclusive.
 13. The method according to claim 1,wherein the liquid contains a solute.
 14. The method according to claim1, wherein a proportion of water in the liquid ranges from 1.0% by massto 5.0% by mass, inclusive.
 15. The method according to claim 1, whereinthe first layer is conductive.
 16. The method according to claim 1,wherein the liquid is an electrolyte solution.